1. Field of the Invention
This invention relates to electronic security systems and especially to such systems which are designed to reduce or eliminate inadvertent alarm actuations in response to interference signals.
2. Discussion of Related Art
Electronic security systems are known which detect the presence of a resonant tag circuit which may be attached to an article. Such systems are especially useful to prevent theft in retail stores, and the unauthorized removal of books or documents from a secure location, or the like. However, such systems are known to be susceptible to producing a false alarm when interfering noise signals are present in the vicinity. An inadvertent alarm can cause embarrassment in a retail store environment by prompting security personnel to detain a shopper who may coincidentally be passing the security system at the time of the alarm. Further, an inadvertent alarm gives notice to persons in the vicinity of the existence of a security system which may lead to a knowledgeable thief taking steps to avoid detection. Consequently, a need has arisen for noise rejection circuitry which is readily adapted for use in an electronic security system.
Noise rejection circuitry has been suggested in the past. For example, U.S. Pat. No. 3,828,337 to Lichtblau discloses such circuitry in which true signals are distinguished from noise by sensing the absence of one or more pulses in an expected train of pulses produced by the resonant tag. The Lichtblau patent is deficient in that timing circuits are required which must be within certain tolerances. If these tolerances vary, the circuitry operation degrades drastically.